Automotive vehicles with automatic transmissions rely on a hydraulic pump to provide fluid pressure to operate clutches internal to the transmission during transmission operation. However, the hydraulic pump does not operate when the vehicle engine is stopped. To enhance fuel economy, some vehicles with automatic transmissions stop the engine when the vehicle comes to a stop. In order to prevent a delay while pressure builds within the fluid lines, the transmission may be equipped with a clutch that is normally engaged, i.e., the clutch is engaged to transfer torque in the absence of fluid pressure from the main hydraulic pump. The normally engaged clutch positions the transmission in a first or low gear in preparation for launch, so that as soon as the engine is restarted from the automatic start/stop, torque may be transferred through the transmission to the wheels without waiting for the fluid pressure to build up within the fluid lines. As soon as pressure within the fluid lines builds, the normally engaged clutch may be controlled as usual to either maintain torque communication, or disengage the clutch.
Because the normally engaged clutch transfers torque through the transmission, even in the absence of fluid pressure from the main hydraulic pump, and because the normally engaged clutch requires a fluid pressure from the hydraulic pump to disengage and prevent torque transfer through the transmission, it is necessary to identify a failure in the main hydraulic pump and control the vehicle appropriately in the event that the hydraulic pump fails.